Barnali Ghosh

Crystal structure and physical properties of half-doped manganite nanocrystals of less than 100-nm size

In this paper we report the structural and property (magnetic and electrical transport) measurements of nanocrystals of half-doped La 0.5 Ca 0.5 MnO 3 (LCMO) synthesized by chemical route, having particle size down to an average diameter of 15 nm. It was observed that the size reduction leads to change in crystal structure, and the room temperature structure is arrested so that the structure does not evolve on cooling unlike bulk samples. The structural change mainly affects the orthorhombic distortion of the lattice. By making comparison to observed crystal structure data under hydrostatic pressure, it is suggested that the change in the crystal structure of the nanocrystals occurs due to an effective hydrostatic pressure created by the surface pressure on size reduction. This not only changes the structure but also causes the room temperature structure to freeze. The size reduction also does not allow the long supercell modulation needed for the charge ordering, characteristic of this half-doped manganite, to set in. The magnetic and transport measurements also show that the charge ordering (CO) does not occur when the size is reduced below a critical size. Instead, the nanocrystals show ferromagnetic ordering down to the lowest temperatures along with metallic-type conductivity. Our investigation establishes a structural basis for the destabilization of CO state observed in half-doped manganite nanocrystals.

Photoreactivity of ZnO nanoparticles in visible light: Effect of surface states on electron transfer reaction

Wide band gap metal oxide semiconductors such as zinc oxide (ZnO) show visible band photolysis that has been employed, among others, to degrade harmful organic contaminants into harmless mineral acids. Metal oxides show enhanced photocatalytic activity with the increase in electronic defects in the crystallites. By introducing defects into the crystal lattice of ZnO nanoparticles, we observe a redshift in the optical absorption shifting from the ultraviolet region to the visible region (400-700 nm), which is due to the creation of intermediate defect states that inhibit the electron hole recombination process. The defects were introduced by fast nucleation and growth of the nanoparticles by rapid heating using microwave irradiation and subsequent quenching during the precipitation reaction. To elucidate the nature of the photodegradation process, picosecond resolved time correlated single photon count (TCSPC) spectroscopy was carried out to record the electronic transitions resulting from the de-excitation of the electrons to their stable states. Photodegradation and TCSPC studies showed that defect engineered ZnO nanoparticles obtained through fast crystallization during growth lead to a faster initial degradation rate of methylene blue as compared to the conventionally synthesized nanoparticles.

Evidence of a canted magnetic state in self-doped LaMnO3+δ (δ = 0.04): a magnetocaloric study

We report a detailed investigation of the magnetocaloric properties of self-doped polycrystalline LaMnO(3+δ) with δ = 0.04. Due to the self-doping effect, the system exhibits a magnetic transition from a paramagnetic to ferromagnetic-like canted magnetic state (CMS) at ~120 K, which is associated with an appreciably large magnetocaloric effect (MCE). The CMS is an inhomogeneous magnetic phase developing due to a steady growth of antiferromagnetic correlation in its predominant ferromagnetic state below ∼120 K. The stabilization of CMS in this material is concluded from a comprehensive analysis of magnetocaloric data using Landau theory, which is in excellent agreement with our neutron diffraction study. The magnetic entropy change versus temperature curves for different applied fields collapse into a single curve, revealing a universal behavior of MCE. Our studies suggest that investigation of MCE is an effective technique to acquire fundamental understanding about the basic magnetic structure of a system with complex competing interactions.

Functionalization of manganite nanoparticles and their interaction with biologically relevant small ligands: Picosecond time-resolved FRET studies

We report molecular functionalization of the promising manganite nanoparticles La0.67Sr0.33MnO3 (LSMO) for their solubilization in aqueous environments. The functionalization of individual NPs with the biocompatible citrate ligand, as confirmed by Fourier transform infrared (FTIR) spectroscopy, reveals that citrates are covalently attached to the surface of the NPs. UV-VIS spectroscopic studies on the citrate functionalized NPs reveals an optical band in the visible region. Uniform size selectivity (2.6 nm) of the functionalization process is confirmed from high resolution transmission electron microscope (HRTEM). In the present study we have used the optical band of the functionalized NPs to monitor their interaction with other biologically important ligands. Förster resonance energy transfer (FRET) of a covalently attached probe 4-nitrophenylanthranilate (NPA) with the capped NPs confirm the attachment of the NPA ligands to the surface functional group (-OH) of the citrate ligand. The FRET of a DNA base mimic, 2-aminopurine (2AP), with the NPs confirms the surface adsorption of 2AP. Our study may find relevance in the study of the interaction of individual manganite NPs with drug/ligand molecules.

Growth and physical property study of single nanowire (diameter ∼45nm) of half doped manganite

We report here the growth and characterization of functional oxide nanowire of hole dopedmanganite of La0.5Sr0.5MnO3 (LSMO). We also report four-probe electrical resistance measurement of a single nanowire of LSMO (diameter ∼45 nm) using focused ion beam (FIB) fabricated electrodes. The wires are fabricated by hydrothermal method using autoclave at a temperature of 270 °C. The elemental analysis and physical property like electrical resistivity are studied at an individual nanowire level. The quantitative determination ofMn valency and elemental mapping of constituent elements are done by using Electron Energy Loss Spectroscopy (EELS) in the Transmission Electron Microscopy (TEM) mode. We address the important issue of whether as a result of size reduction the nanowires can retain the desired composition, structure, and physical properties. The nanowires used are found to have a ferromagnetic transition (TC) at around 325K which is very close to the bulk value of around 330K found in single crystal of the same composition. It is confirmed that the functional behavior is likely to be retained even after size reduction of the nanowires to a diameter of 45 nm. The electrical resistivity shows insulating behavior within the measured temperature range which is similar to the bulk system.

Electronic transport in nanostructured films of La0.67Sr0.33MnO3

In this paper we report electronic transport in nanostructured films of the rare-earth manganite La0.67Sr0.33MnO3. The films were grown by chemical solution deposition. The films show a resistivity peak in the temperature range of 250–265 K and have average grain size (∼50–60nm). The grain size can be controlled by postdeposition annealing. The films also show a rise in resistivity at low temperature (T<40K), reasonable low-field magnetoresistance up to 200 K, and nonlinear conductivity that shows up below 30 K. We ascribe these behaviors to the large number of natural grain boundaries that are present in these nanostructured films. We were also able to map the inhomogeneous local electronic properties arising from these grain boundaries using a variable-temperature scanning-tunneling microscope. We found that as the temperature is lowered, due to differences between the electronic properties of the grains and grain boundaries, the transport becomes more inhomogeneous. The nonlinear conduction as well as ...

Phase coexistence and magnetic anisotropy in polycrystalline and nanocrystalline LaMnO3+δ

We report on the phase coexistence and magnetic anisotropy in polycrystalline (bulk) and nanocrystalline (∼15 nm) LaMnO3+δ materials, which were prepared by solid state reaction and sol-gel methods, respectively. In addition to standard magnetization measurements, radio-frequency transverse susceptibility (TS) based on a very sensitive, self-resonant tunnel diode oscillator method was used to probe magnetic anisotropy and switching fields in the samples. The results revealed a coexistence of the ferromagnetic (FM) and antiferromagnetic (AFM) phases in both samples. For the bulk sample, the AFM phase significantly changed in volume fraction at ∼30 K and completely vanished around 120 K. Size reduction to the nanometer scale (∼15 nm) significantly suppressed the AFM phase while inducing surface spin disorder in the material. The large magnetic anisotropies were probed by TS experiments in both samples. Our studies showed that the magnetic properties of bulk LaMnO3+δ were strongly modified by size reduction.

Inverse magnetocaloric and exchange bias?effects in single crystalline La0.5Sr0.5MnO3 nanowires

We report the first observation of inverse magnetocaloric effect (IMCE) in hydrothermally synthesized single crystalline La0.5Sr0.5MnO3 nanowires. The core of the nanowires is phase separated with the development of double exchange driven ferromagnetism (FM) in the antiferromagnetic (AFM) matrix, whereas the surface is found to be composed of disordered magnetic spins. The FM phase scales with the effective magnetic anisotropy, which is directly probed by transverse susceptibility experiments. The surface exhibits a glassy behavior and undergoes spin freezing, which manifests as a positive peak (T(L) ~ 42 K) in the magnetic entropy change (-ΔS(M)) curves, thereby stabilizing the re-entrance of the conventional magnetocaloric effect. Precisely at T(L), the nanowires develop the exchange bias (EB) effect. Our results conclusively demonstrate that the mere coexistence of FM and AFM phases along with a disordered surface below their Néel temperature (T(N) ~ 210 K) does not trigger EB, but this develops only below the surface spin freezing temperature.

Residual gas analyzer mass spectrometry for human breath analysis: a new tool for the non-invasive diagnosis of Helicobacter pylori infection

A residual gas analyzer (RGA) coupled with a high vacuum chamber is described for the non-invasive diagnosis of the Helicobacter pylori (H. pylori) infection through ¹³C-urea breath analysis. The present RGA-based mass spectrometry (MS) method is capable of measuring high-precision ¹³CO₂ isotope enrichments in exhaled breath samples from individuals harboring the H. pylori infection. The system exhibited 100% diagnostic sensitivity, and 93% specificity alongside positive and negative predictive values of 95% and 100%, respectively, compared with invasive endoscopy-based biopsy tests. A statistically sound diagnostic cut-off value for the presence of H. pylori was determined to be 3.0‰ using a receiver operating characteristic curve analysis. The diagnostic accuracy and validity of the results are also supported by optical off-axis integrated cavity output spectroscopy measurements. The δ¹³(DOB)C‰ values of both methods correlated well (R² = 0.9973 at 30 min). The RGA-based instrumental setup described here is simple, robust, easy-to-use and more portable and cost-effective compared to all other currently available detection methods, thus making it a new point-of-care medical diagnostic tool for the purpose of large-scale screening of the H. pylori infection in real time. The RGA-MS technique should have broad applicability for ¹³C-breath tests in a wide range of biomedical research and clinical diagnostics for many other diseases and metabolic disorders.

Non-invasive 13C-glucose breath test using residual gas analyzer-mass spectrometry: a novel tool for screening individuals with pre-diabetes and type 2 diabetes

We report, for the first time, the clinical feasibility of a novel residual gas analyzer mass spectrometry (RGA-MS) method for accurate evaluation of the (13)C-glucose breath test ((13)C-GBT) in the diagnosis of pre-diabetes (PD) and type 2 diabetes mellitus (T2D). In T2D or PD, glucose uptake is impaired and results in blunted isotope enriched (13)CO2 production in exhaled breath samples. Using the Receiver operating characteristics (ROC) curve analysis, an optimal diagnostic cut-off point of the (13)CO2/(12)CO2 isotope ratios expressed as the delta-over-baseline (DOB) value, was determined to be δDOB(13)C‰ = 28.81‰ for screening individuals with non-diabetes controls (NDC) and pre-diabetes (PD), corresponding to a sensitivity of 100% and specificity of 94.4%. We also determined another optimal diagnostic cut-off point of δDOB(13)C‰ = 19.88‰ between individuals with PD and T2D, which exhibited 100% sensitivity and 95.5% specificity. Our RGA-MS methodology for the (13)C-GBT also manifested a typical diagnostic positive and negative predictive value of 96% and 100%, respectively. The diagnostic accuracy, precision and validity of the results were also confirmed by high-resolution optical cavity enhanced integrated cavity output spectroscopy (ICOS) measurements. The δDOB(13)C‰ values measured with RGA-MS method, correlated favourably (R(2) = 0.979) with those determined by the laser based ICOS method. Moreover, we observed that the effects of endogenous CO2 production related to basal metabolic rates in individuals were statistically insignificant (p = 0.37 and 0.73) on the diagnostic accuracy. Our findings suggest that the RGA-MS is a valid and sufficiently robust method for the (13)C-GBT which may serve as an alternative non-invasive point-of-care diagnostic tool for routine clinical practices as well as for large-scale diabetes screening purposes in real-time.